1. Field of Invention
The present invention relates to a down hole tool for enlarging a drill bore; and more specifically it relates to a reamer that enlarges a pilot hole.
2. State of the Art
It has long been known by those involved in drilling subterranean bore holes for oil and gas exploration, mineral recovery or in utility construction projects, to employ reamers to enlarge bore holes after a smaller pilot hole has been drilled. Pilot holes can be drilled economically and more precisely. Pilot holes can also be drilled with down hole motors that can be guided to the desired location from the point of ingress of the drill string.
Prior reamers have heretofore been manufactured in several distinct styles. One type of reamer provides extensible arms and cutters that are used to enlarge the hole to a larger diameter by rolling and crushing the face and sides of the borehole with hardened buttons or dressed teeth. It is well known in the drilling industry that these buttons or dressed teeth can be fabricated from materials such as tungsten carbide or polycrystalline diamond compact (PDC) or the like. The second type of reamer or hole opener consisted of a tubular body with a plurality of arms supporting journals and cutter bodies to extend to and crush the well bore. These arms, whether extensible or fixed, and the smaller cutter cones carried on those arms, are subject to failure in the hole as a result of their limited bearing surfaces and the smaller buttons, which wear out prematurely. As the number of arms increases, the size of the bearings that can be used to provide rotational movement of the cutter around each support arm diminishes, limiting the life thereof. Further, because the extensible arms and cutter bodies were held next to the body while going into the borehole, material was removed from the tubular body further weakening the integrity of the entire tool. Other types of reamers or hole openers have been fashioned with three or more fixed cones on journals mounted on integral posts on a tubular body. These hole openers or reamers were similarly limited in the size of the cones and bearings supported. The small cutter buttons and bearing surfaces cause short service lives and can lead to premature failure of the tool. When failure occurred down hole for either type of reamer (extensible arm or fixed arm), substantial time and effort was required to fish the tool from the bore or to drill around the tool to complete the drilling program.
As noted above, prior art reamers generally provide a plurality of cutter posts and journals onto each of which a cone having tungsten carbide buttons is mounted. Because at least three cones are used on either type of reamer, the cones were of limited size and typically provide small surfaces to contact the borehole face to be reamed. Since smaller hardened buttons (or less hard facing—which alternatively can be used to dress the surface of such cutters) are used to cut the well bore surface, the service life of the reamers is shorter than it would be for tool providing a larger overall bearing surface disposed with larger hardened button cutters. The life of these tools requires that the cutters and journals be redressed to continue their useful life. This redressing historically required the tool to be taken out of service and returned to the shop for repair and reconstruction.
Further, the normal operating problems of conventional hole openers is xacerbated when drilling horizontal or near horizontal applications. In such situations, the load and wear characteristics coming to bear on the support arms can cause early and catastrophic failure of the arm structure and often results in loss of cutters in the borehole itself. Additionally, in horizontal or near horizontal applications, the support arms of conventional openers create additional torque on the tubular string that carries the reamer. The additional torque slows drilling progress and makes the cost per foot of opened hole rise.
These problems can cause failure of both the support arms and loss of cutters in the hole requiring expensive retrieval operations and delay the completion of the operation.
A new type of reamer has long been sought which provides a longer service life because it provides large bearing surfaces and large tungsten carbide buttons or hard facing, which could be used in normal drilling operations to open previously drilled pilot holes. Prior art reamers could not be used under conditions which resulted in substantial longitudinal loading. In utility construction drilling projects, such as river crossings where bore holes are drilled under rivers to permit the installation of utility pipelines, it is often useful to drill from one side of the river to the other with a pilot drill, then ream the hole going from the pilot hole egress side to the ingress side. Utility construction typically therefore requires a reamer that is pulled with substantial force back through the pilot hole. In most drilling projects, substantial or large amounts of longitudinal loading are generally avoided, thereby preventing excessive torque from being introduced into the drill string.